1. Field of the Invention
This invention relates to color management methods, apparatus and systems.
2. Description of the Related Art
While color management has many goals, in the digital color printing industry, the accurate and the characterization of a color printer""s gamut and the subsequent image color matching has become a necessity. The main problem is that characterizing color printers usually involves measuring a large number of color patches that have been printed by the printer in question. In addition, the information that is obtained by measuring these color patches is only valid for that particular instance of the printer, thus creating the need for a quick printer re-characterization method. Printer characterization systems that use a small number of color patch measurements for ICC profile generation exist, but they fail to give accurate results. A traditional approach to generating ICC profiles and to performing color matching involves linear interpolation schemes that require large profile lookup tables.
An additional important consideration to this art is any accuracy/cost tradeoffs that can be made between the accuracy of a printed color and the cost of the ink that produces that printed color. For example, it is known that a slight color error between a signal-requested color and an ink-produced color can be tolerated because the human eye is incapable of noticing such a slight color error. Any ink cost optimization scheme that uses this inability of the human eye to notice slight color errors would be a valuable contribution to the art.
The nature of color as it relates to the electromagnetic spectrum is shown in FIG. 1. Light is a form of electromagnetic energy or electromagnetic waves. Visible light is a form of electromagnetic energy, very much like ultraviolet, infrared, and radio waves. Electromagnetic waves are characterized by their wavelength, which can vary from kilometers to about a millionth of a millimeter. Light waves have wavelengths of about half a millionth of a millimeter, and the unit used for measuring visible light is the nanometer (nm. 10xe2x88x929 m). FIG. 1 uses a logarithmic scale to illustrate a wide range of wavelengths. Color can be defined as the perceptual results of electromagnetic waves in the visible region of the spectrum, having wavelengths in the region of about 400 nm to 700 nm incident upon the eyes of a human. That is, color exists only in the human brain.
Humans can see colored objects due to the three different types of cones that are present with the retina of the human eye. These cones are referred to as S, M, and L (for short, medium and long wavelengths), and have their respective maximum sensitivities at 445 nm (violet), 535 nm (green) and 570 nm (yellow).
Distribution of these three types of cones is not uniform over the eye""s retinal surface, and the number of S cones is much less than the number of the other two types of cones. More specifically, the cone ratios are 1 to 20 to 40, respectively, for the S, M and L cones.
A white surface, such as a piece of white paper, usually has the same visual appearance under both tungsten light and fluorescent light. This phenomenon is termed color constancy, and is due to the ability of the human brain to compensate for changes in both the level and the color of lighting. Sometimes extreme changes in the color appearance of an illumination source may cause detectable changes in the visual appearance of colors. Nevertheless, color constancy is an important feature of human vision.
Three basic perceptual attributes that characterize a color stimulus are: Brightness: Attribute of a color by which it seems to exhibit more or less light: Hue: Attribute of a color that causes the color to be perceived as being other than black, white, or gray; and Saturation: Attribute of a color by which it appears to be pure and containing no white or gray.
The term lightness is typically used to describe the brightness of a color relative to that of an equally illuminated white background. If illumination intensity changes, then both the color and the white background change equally, thereby maintaining the same lightness. In addition, colorfulness can be judged in proportion to the brightness of white, and is termed chroma. Lightness and chroma are, therefore, defined as: Lightness: The brightness (light/dark) of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting; and Chroma: The colorfulness (strong/weak) of an area judged in proportion to the brightness of a similarly illuminated area that appears to be white or highly transmitting.
Colorfulness can also be judged relative to the brightness of the same area, instead of to the brightness of a white area. This again is saturation. These definitions only apply to related colors; that is, to colors that are viewed together under the same conditions. Unrelated colors, on the other hand, are colors that are isolated from each other.
In the early years of color science, colored surfaces were compared by placing them side by side under a standard light source. This methodology was used since little was known about the human eye and its functions. Today, color science has a good understanding of the eye""s cone response, and of how the cones interact to produce a sensation of color within the human brain. A software or hardware model of color must take into account a knowledge of the human mechanism of color vision.
Since humans have the three types of cones S, M, and L, and since photopic vision and color vision are a function of three variables, it is expected that an apparatus/method/system for the evaluation of color from spectral power data would require three different weighting functions.
An internationally-accepted method for evaluating color is called trichromic color matching or three color matching. FIG. 2 shows a basic experimental arrangement for trichromatic color matching. In FIG. 2, the target color to be matched is projected on the lower half of the eye""s field of view, while, on the other half of the filed of view, a mixture of three lights or stimuli is projected. The three projected light stimuli are usually 700 nm (red), 546.1 nm (yellowish green) and 435.8 nm (bluish violet). Color matching is then achieved by changing the intensities of the red, green, and blue source, until the target color is matched.
Many objects appear to be colored by absorbing and/or reflecting portions of the light that is incident on them, rather than by emitting light themselves. It is, therefore, clear that both the quality and the type of light affects a human viewer""s perception of a reflective color. Humans are exposed to several light sources that have very diverse spectral characteristics. For example, incandescent light bulbs (light is emitted by a heated filament often made of tungsten) tend to distribute the majority of their energy to the longer wavelengths. This characteristic makes incandescent light appear more yellow than natural day light, which light is fairly neutral.
Any illuminant source, therefore, can be defined as a special kind of light source that is defined in terms of its spectral characteristics. Since most sources of illumination produce light by heating an object, these sources can be characterized by specifying the temperature of a black body radiator that appears to have the same hue.
Color temperature Tc (in units of Kelvin, K) is defined as the temperature at which a Plankian black body radiates light that has the same chromaticity as that of a given light source. This definition is only applied to light sources that are very similar to Plankian radiators. The Correlated Color Temperature Tcp is defined as the Plankian radiator temperature that most closely approximates the hue of the light source in question. This second definition applies to sources that do not exactly resemble a Plankian radiator.
The CIE standard illuuminant A is used to simulate incandescent lighting and it is created using a tungsten light source with a color temperature of 2856 K. The CIE standard illuminant B is used to simulate direct noon sunlight, neutral in hue, by filtering a tungsten light source with a color temperature of 4874 K. The CIE standard illuminant C is used to simulate average daylight by again filtering a tungsten light source with a color temperature of 6774 K. The filter consists of a tank of blue liquid, the chemical composition of which is specified by the CIE. These three CIE standard illuminants are defined within the wavelength range of 380-700 nm, and do not include the ultraviolet region that is needed to accurately describe fluorescent colors.
In 1963, the CIE recognized deficiencies of the first three standard illuminants, and defined a new family of light sources called the D series illuminants. Each such illuminant is given the name D with a two digit subscript to indicate the color temperature of the illuminant. The most common D illuminants in the graphic arts industry corresponds to a temperature of 5000 K, and are thus termed D50. The spectral distribution of a D50 illuminant is very difficult to reproduce with an artificial light. There is no actual apparatus specified as part of the CIE standard. Instead, there is a procedure to determine how well a given light source matches the color matching properties of the D50 illuminant. Illuminant E is a theoretical illuminant with a nearly flat spectral profile. Illuminant E light does not exist, but it is used to perform computations in color science. F series illuminants are designed to simulate fluorescent lighting. Fluorescent lamps consist of a glass tube filled with low pressure mercury gas, coated on the inside with phosphors. The mercury gas is charged by an electrical current which produces ultraviolet radiation. The phosphors then absorbs the radiation and re-emits the radiation at a broader range of wavelengths in the visible part of the spectrum. The process of re-emitting incident radiation at different wavelengths is called fluorescence. There are twelve different F illuminants, differing in the type of phosphors used. The CIE recommends F2, F7 and F11 as reasonable illuminants for evaluating colors that will be viewed under fluorescent lighting environments. The spectral profile of a fluorescent light is easily distinguished by the sharp spikes caused by the mercury gas.
There are several ways to calculate the spectrum and, therefore, the tristimulus values of a light source. Methods, such as visual color matching and calculation from spectral data can be used. A first method uses additive mixtures of red, green and blue light. This method is reliable, but it is time consuming and is usually not considered for tristimulus value determinations. A second method requires the CIE color matching functions, source spectral data, and illuminant spectral data in some cases. Obtaining this spectral data involves spectroradiometry and spectrophotometry.
Since color perception is a complicated process, simple models are defined in order to describe color, using the least number of parameters. Typically, color spaces use three primary colors from which others are obtained by mixture. In other cases, color is defined using parameters, such as lightness, hue and saturation.
Color spaces can be classified into device dependent (RGB, CMYK) and device independent (LAB, XYZ, etc.). RGB and CMYK color spaces are termed xe2x80x9cdevice dependentxe2x80x9d because two different color video monitors, or two different color printers reproduce the same percentages of RGB or CMYK very differently, depending on the monitor phosphor or printer ink (toner) that is used.
Device-independent color spaces communicate color accurately since they do not use device color primaries directly. The only disadvantage of the device-independent approach is that mathematical transformations and interpolations are usually required.
The RGB color space is described as a combination of the three additive (adding light to darkness) primaries red, green, and blue. The RGB color space is used for devices that mimic the trichromtic properties of the human eye, such as color monitors, color scanners and color projectors.
The CMYK color space describes color as a combination of the three subtractive (removing light from white) primaries cyan, magenta, and yellow. Color printers use this color space by applying layers of CMY ink onto white paper. A fourth black primary color black (K) is typically used to reduce the total amount of ink that is required in order to produce a given color. For example, a neutral gray color can be produced by using 2 units of each of the three CMY primaries (total of 6 units), or this same neutral gray color can be produced by using 1 unit each of the three CMY primaries, and 1 unit of black (total of 4 units).
In the XYZ color space of FIG. 3, the three XYZ tristimulus values define a three-dimensional color space that encloses all visible colors. In certain occasions, it is useful to examine colors by removing the lightness component, thereby, mapping a XYZ solid area 11 onto a two-dimensional plane 12. To achieve this, the tristimulus values are normalized.                     x        =                  x                      X            +            Y            +            Z                                                  y        =                  y                      X            +            Y            +            Z                                                            z          =                      z                          X              +              Y              +              Z                                      ⁢                  
                ⁢                              x            +            y            +            z                    =          1                    
Since all primaries add to unity, it is customary to drop the z term. The resulting two-dimensional diagram is called the CIE 1931 chromaticity diagram, and is shown in FIG. 4. In the chromaticity diagram of FIG. 3, only hue and saturation are represented. The curved line in FIG. 3 indicates where all single wavelength colors of the spectrum lie, and this line is known as the spectral locus. If two color stimuli C1, C2 are additively mixed together, then the point C3 representing the mixture will always lie on a line that connects the two colors, as shown in FIG. 3. The line 14 that connects the 450 nm color and the 650 nm color is an artificial boundary, termed the purple boundary, and represents all of the colors seen when blue and red are mixed together.
A color space is called uniform, or approximately uniform, when equal distances correspond to equally perceived color differences (as seen by the human eye). The XYZ and the xy color spaces of FIGS. 3 and 4, respectively, accurately represent color, but they lack perceptual uniformity.
For example, FIG. 4 shows the xy chromaticity coordinates of colors that correspond to five equal increments of cyan, magenta and yellow. It can be seen that while this CMY color space is relatively uniform, the xy and, therefore, the XYZ spaces, are far from it. A mathematical transformation is needed for the XYZ spaces to exhibit reasonable perceptual uniformity.
Two transformations, called the CIELUV and CIELAB, reduce the non-uniformity of the XYZ space from 80:1 to about 6:1. If the luminance Y of a given color is divided by the luminance of a reference white, a relative scale from 0 to 100 is defined. It can be seen that the measured luminance does not correspond to the perceived lightness, since the majority of the dark colors are found at one end of the scale.
The CIELAB color space color model addresses the problem of perceptual non-uniformity by transforming the XYZ color space. The result is a three-dimensional, approximately uniform, color space having three orthogonally arranged axes L, a, b, respectively numbered 15, 16, 17 in FIG. 5. As shown in FIG. 5, the L axis 15 extends between black (L=0) and white (L=100), the a axis 16 extends between red (a+) and green (axe2x88x92), and the b axis 17 extends between yellow (b+and blue (bxe2x88x92). Colors whose coordinates lie adjacent to origin 18 are pale, and saturation increases as the coordinates of a color lie further away from origin 18.
It is important to graphic arts and the color output device industry that accurate instrumentation be available for describing and communicating color information. Three main types of color measurement instruments are densitometers, calorimeters and spectrophotometers.
A densitometer (unlike a spectrometer) measures color density; that is, the ability of a surface to absorb incident light. Reflectance and density are usually inversely proportional to each other in that as the reflectance of a surface decreases, the color density of the surface increases. Since measured reflectivity values can vary from almost zero to 1, this reciprocal relationship provides color density values ranging from zero to infinity. This large dynamic range of values can be condensed to a more reasonable range by taking the logarithm of the ratio which is an industry standard known as log density.
Densitometers, very much like spectrophotometers, have built in response functions that describe the sensitivity of the instrument at each wavelength. For instance, if the color density of cyan ink needs to be calculated, the densitometer needs to determine how much red is absorbed, therefore, isolating the cyan from the other inks that are present. The American National Standards Institute has specified a series of response curves, termed the ANSI Status classifications, depending upon the material to be measured. The most common ANSI classifications are Status T (reflective) used for measuring graphic arts production materials, such as press proofs, off-press proofs and press sheets, Bf Status E (reflective) European choice over Status T, Bf Status A (reflective or transmissive) used for measuring color photographic materials, such as photographic prints, 35 mm slides, etc.
Colorimeters are devices that measure color similar to the way in which the human eye perceives color. These devices measure the tristimulus values of a stimulus by using three sensors that have response curves similar to the response curves of the cones within the human eye. The majority of calorimeters report the color information using CIE color spaces, such as XYZ, xyY, Luv, or LAB (Lab). Most calorimeters can also measure and report xcex94E values between a given color and a measured color.
A spectrophotometer measures spectral data for a target color. This is done by dividing the visible spectrum from 390 nm to 700 nm into small discrete bands. The width of each band in nanometers is called its bandwidth. Typically, a constant bandwidth of 10 nm (32 band) is sufficient for capturing color information. There are spectrometer devices that have non-uniform bandwidths, called variable bandwidth spectrophotometers. Since spectral data can be converted to tristimulus values, spectrophotometers can usually report both colorimetric and color density values. Since some computation is required to obtain calorimetric data from spectral data, a computer or an internal processor is usually a part of the device, thus making spectrophotometers the most expensive of the three devices. In calculating tristumulus values from spectral data, visible spectrum is first divided into small wavelength intervals. For each interval, the average intensity of the color is calculated, as well as the tristimulus values for each wavelength, using color matching functions. The overall XYZ values are then calculated using the equation:
X=K(Pi{overscore (x)}1+P2{overscore (x)}2+P3{overscore (x)}3 . . . )
Y=K(Pi{overscore (y)}1+P2{overscore (y)}2+P3{overscore (y)}3 . . . )
Z=K(Pi{overscore (z)}1+P2{overscore (z)}2+P3{overscore (z)}3 . . . )
where K is a constant value that normalizes Y to 1.
For reflective colors, the power P values are calculated by multiplying their reflectance factors with the spectral power distribution of the light source. A spectrophotometer uses a photocell to measure the brightness at each wavelength interval, and has standard observer color matching functions and illuminant spectral functions in memory in order to perform calculations.
Metameric stimuli are color stimuli that have the same tristimulus values and different spectral distributions for the same illuminant and the same human observer. This phenomenon is called metamerism. Color printers rely on metamerism to substitute equal percentages of the CMY inks with black ink, and still maintain the same color values for the new CMYK ink.
FIG. 6 illustrates how the spectral information 19 that is contributed by an illumination source, the spectral information 20 that is contributed by the surface on which an image is printed, and the spectral information 21 that is contributed by the tristimulus response of the eye of a human viewer can be combined at 22 to form a XYZ color space 23, such as shown in FIG. 5, which color space 23 can then be converted to xyZ, LAB or LUV color spaces 24.
The present invention relates to the use of a relatively small number of carefully selected and individually different images of one unique color that have been produced by a color printer to train a gamut-defining network, such as neural network. In a first embodiment of the invention, this gamut-defining network is used to generate a printer profile lookup table for use in a color management system, wherein an input image signal controls the color printer to print in accordance with the content of the lookup table. In a second embodiment of the invention, the gamut-defining network is directly used to control printing by the color printer, so as to produce color prints in accordance with a comparison of the input image signal to the printer""s gamut.
The present invention fills a need that remains in the art for a system that uses a small number of color patch measurements, provides accurate printer characterization, generates ICC profiles, and performs color management.
The present invention provides a new and unusual optimization theory based Color Management System (CMS). One aspect of the invention comprises a CMS tool that uses artificial neural networks for performing efficient color space conversion, and uses optimization to perform printer gamut mapping and gray component replacement, it being recognized that others have taught the general use of neural networks in CMS.
This invention provides a new and unusual approach to performing color management, and to generating International Color Consortium (ICC) printer profiles, while at the same time, reducing the number of measurements that are required. Color printer characterization and the generation of accurate ICC printer profiles usually requires a large number of measured data points, and complex interpolation techniques. This invention provides a neural network representation and optimization based apparatus and method for performing color space conversion, printer gamut mapping and gray component replacement.
A neural network architecture is provided for performing CMYK to LAB color space conversion for a color printer, such as a CMYK IBM Network color printer. A feed forward neural network is trained using an ANSI/IT-8 basic data set, consisting of 182 data points, or using a reduced data set of 150 or 101 data points when linear portions of the 182 data set are eliminated. A 5-to-7 neuron neural network architecture is preferred for color space conversion purposes. Neural network functions and optimization routines are used to perform printer gamut mapping and gray component replacement.
This invention provides novel optimization and artificial neural network models that carry out color space conversion, printer gamut mapping and gray component replacement. Features of the present invention include: (1) Performing color space characterization using a relatively small number of color patch measurements; (2) Using a single step color management process; (3) Performing accurate color management; (4) Operating as a stand alone, image specific, color management system; (5) Generating ICC printer profiles that conform to the ICC specification; (6) Allowing easy printer calibration; and (7) Allowing very different rendering intent to be handled in a convenient manner.
In a the method in accordance with this invention, (1) a relatively small number of carefully selected and different color patch prints are produced using a CMYK color printer, (2) these color patch prints are then used to train a gamut-defining network, such as a neural network, preferably comprising from 5 to 7 neurons, thereby defining the gamut of the CMYK printer, as this gamut is defined by the relatively small number of color panel prints. In a first embodiment of the invention, this trained gamut-defining network is used to generate a printer profile lookup table that converts LAB values to corresponding CMYK values in accordance with the gamut of the CMYK printer. In a second embodiment of the invention, the apparatus/method is used to directly control a CMYK printer, wherein LAB input image signals control the CMYK color printer in accordance the gamut-defining network.
In accordance with an important feature of this invention, the gamut-defining network is used in combination with a non-linear optimization procedure to map any image color to a CMYK value that is within the printer""s gamut.
When a lookup table is being constructed in accordance with this invention, a uniform grid of LAB values are inputted so that these LAB values can be mapped to CMYK values that are within the printer""s gamut, and a lookup table is thereby embedded in an ICC profile that converts LAB image information into a CMYK image that best approximates the LAB image information for the printer under consideration.
When a printer is being directly controlled in accordance with this invention, the LAB image information input to the gamut-defining network, and this network operates convert LAB image information into a CMYK image that best approximates the LAB image information for the printer under consideration.
These and other features and advantages of the invention will be apparent to those of skill in the art upon reference to the following detailed description, which description makes reference to the drawing.